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Added warning about same-address-optimization
See https://github.com/pybind/pybind11/issues/254
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@ -468,7 +468,7 @@ functions. The default policy is :enum:`return_value_policy::automatic`.
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| :enum:`return_value_policy::take_ownership` | Reference an existing object (i.e. do not create a new copy) and take |
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| | ownership. Python will call the destructor and delete operator when the |
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| | object's reference count reaches zero. Undefined behavior ensues when the |
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| | C++ side does the same.. |
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| | C++ side does the same. |
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+--------------------------------------------------+----------------------------------------------------------------------------+
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| :enum:`return_value_policy::copy` | Create a new copy of the returned object, which will be owned by Python. |
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| | This policy is comparably safe because the lifetimes of the two instances |
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@ -526,6 +526,15 @@ The following example snippet shows a use case of the
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non-determinism and segmentation faults, hence it is worth spending the
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time to understand all the different options in the table above.
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.. warning::
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pybind11 tries to eliminate duplicate addresses by returning the same reference object.
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If two addresses are the same, though they do not point to the same object semantically,
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this may cause unexpected behaviour. An explicit policy should be used instead of
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relying on `automatic`.
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A common example is a reference to the first member of a class which has the same memory
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location as its owning class.
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.. note::
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The next section on :ref:`call_policies` discusses *call policies* that can be
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